1. Field of the Invention
This invention relates to methods of using an alternating magnetic field (AMF) to generate hysteresis losses in multi-phase nanocrystalline metal-aluminum powders. When the powders are subjected to an AMF of appropriate strength and frequency for a suitable period, an exothermic reaction is triggered, which causes the powders to melt. A metal aluminide, having an ordered crystal structure, solidifies from the melt. The metal aluminide does not exhibit hysteretic behavior and cannot be melted by application of an AMF.
2. Description of the Related Art
The most common techniques for joining parts are soldering, brazing, and welding, which involve heating and liquifying metals or metal alloys at a seam or junction where two or more parts are to be joined. Heat is typically applied by a resistively heated solder gun or an acetylene torch. However, such ‘brute force’ heating methods often cause heating, not only of the solder or braze, but also of the parts to be joined, which may lead to dimensional distortion and changes in physical and/or mechanical properties.
In an attempt to provide more precise heating, methods of using AMFs to induce a flow of current and/or generate magnetic hysteresis losses within a joining material have been developed. However, each of these mechanisms has practical limitations when applied to conventional joining materials. For example, induction heating requires that a joining material be conductive and arranged to provide a conduction path. For this reason, it is generally not possible to inductively heat powders, which may be physically separated and/or coated with a surface oxide that prevents current flow between particles.
On the other hand, when heating via magnetic hysteresis losses the maximum achievable temperature is the Curie temperature of the magnetic material. Typically, the maximum achievable temperature (Curie temperature) is sufficient to melt polymeric materials, but not metals. Further, bonds formed between parts may be weakened by exposure to extraneous magnetic fields that can cause unintended reheating of the joining material.
Metal oxide particles dispersed within a polymer matrix represent one example of a known system that provides joining of parts via magnetic hysteresis losses. However, metal oxide systems suffer from the disadvantages discussed above, namely: (1) the Curie temperature of the metal oxide represents a maximum achievable temperature; (2) the matrix (polymer) material must have a melting point that is less than or equal to the Curie temperature of the metal oxide, which is typically between about 100 to 600° C.; (3) the metal oxide particles remain ferromagnetic within the resolidified polymer allowing the polymer to reheat and melt if subjected to another AMF; and (4) polymer bonds are typically weaker than metal or alloy bonds.
Another approach to obtaining precise heating involves the use of Ni—Al multilayer foils, which are deposited by thin film deposition techniques, between parts to be joined. The Ni—Al foils are ignited by a flame or electrical impulse. Following ignition, a nickel aluminide (NiAl) compound having an ordered B2 structure solidifies and bonds the parts together. This process provides precise heating and rapid bond formation. However, it is expensive to perform thin film deposition, and the use of thin film deposition generally limits the parts to be joined to those having planar surfaces.